This invention relates to a binder for an electrode comprising a vinyl alcohol polymer, a slurry thereof, an electrode made by using the slurry and a non-aqueous electrolyte secondary battery.
In recent years portable instruments such as video cameras, portable telephones and personal computers have spread wide. Thus, there is an increasing demand for secondary batteries, which can be repeatedly used, in substitution for primary batteries which are disposable.
Among the currently available secondary batteries, nickel cadmium batteries with an alkali electrolyte are most widely used. However, nickel cadmium batteries have problems such that the voltage generated is low and thus the energy density is not enhanced, and the self-discharge is large.
Non-aqueous electrolyte secondary batteries with a negative electrode made of light metal such as lithium attract attention because of high energy density, reduced self-discharge and lightness in weight. However, the secondary batteries with a lithium negative electrode have poor practicality because, with repetition of charge and discharge, metallic lithium crystal develops in the form of dendrite and contacts a positive electrode causing short circuit.
To solve this problem, a proposal has been made wherein an alloy of lithium with other metals is used as a negative electrode of a non-aqueous electrolyte secondary battery. This secondary battery also has poor practicality because another problem arises in that the alloy electrode is collapsed into granules with repetition of charge-discharge cycles.
A further proposal has been made wherein a carbonaceous material such as coke or graphite is used as an active material for a negative electrode of a non-aqueous electrolyte secondary battery. In this battery, doping and dedoping of lithium ion in and from a laminar structure of carbon are utilized for the negative electrode reaction, and thus, problems of the deposition of lithium metal or the granulation of lithium alloy occurring with the prior art batteries using lithium metal or lithium alloy as an active material for a negative electrode do not arise and good charge-discharge cycling characteristics are obtained. Further, when the active material for a positive electrode is made of a lithium transition metal oxide represented by the formula: LixMO2 wherein M is at least one transition metal and x satisfies an inequality of 0.05 less than x less than 1.10, battery capacity is enhanced and a non-aqueous electrolyte secondary battery having a high energy density is obtained.
In this proposed non-aqueous electrolyte secondary battery, the carbonaceous material used as an active material for a negative active material generally has a laminar structure and various forms such as scaly, global, fibrous and other indetermined forms. The lithium-transition metal oxide as an active material for a positive electrode also has a laminar crystal structure and an indetermined form. Upon charging, lithium ion is partly dedoped, i.e., deintercalated, from the laminar crystal structure of a positive electrode and is doped, i.e., intercalated, into the laminar structure of carbon in a negative electrode to form an intercalation compound. Upon discharging, lithium ion moves inversely from the negative electrode to the positive electrode.
A metal foil collector made of, for example, copper or aluminum used in positive and negative electrodes has a function of assisting the movement of electric charges of doped or dedoped lithium ion from a positive electrode to a negative electrode, or from the former to the latter, and further minimizing the loss of electric charges occurring during the course of movement, leading enhancement of the charge-discharge cycling characteristics.
The carbonaceous material and lithium transition metal oxide as active materials in a non-aqueous electrolyte secondary battery are usually used in the form of a powder having an average particle diameter of 0.5 to 60 xcexcm, and cannot be employed as they are for the formation of electrode layers on collectors. Thus, various polymeric adhesives are used as a binder for the electrode formation. As the binder, polymeric adhesives having no problems encountered with the above-mentioned conventional non-aqueous electrolyte secondary batteries are desired.
For example, when carbonaceous material is used as an active material for a negative electrode of a non-aqueous electrolyte secondary battery, a carbonaceous material is finely divided into a powder form and the powdery carbon is dispersed together with a binder in water or an organic solvent to form a slurry for a negative electrode and then a collector for the negative electrode is coated with the slurry. Thus, a negative electrode having the negative active material bound onto the surface of a negative collector by a binder is obtained. Similarly, for example, when a lithium transition metal oxide is used as an active material for a positive electrode, the lithium transition metal oxide in a powder form is dispersed together with an electrically conductive material and a binder in water or an organic solvent to form a slurry for a positive electrode and then a collector for the positive electrode is coated with the slurry. Thus, a positive electrode having the positive active material bound onto the surface of a positive collector by a binder is obtained.
As the binder for electrodes, fluoropolymers such as polyvinylidene fluoride and polytetrafluoroethylene are used in view of good resistance to organic solvents (Japanese Unexamined Patent Publication [hereinafter abbreviated to xe2x80x9cJP-Axe2x80x9d] No. H5-62668, JP-A H8-124561 and JP-A H8-157677). However, fluoropolymer binders such as polyvinylidene fluoride have problems such that they have poor binding power or adhesion to an electrode collector and poor retention of an active material, and thus, the active material bound onto the electrode collector is easily separated from the collector at the repetition of charge and discharge, and the capacity of battery is reduced to a considerable extent with relatively small times repetition of charge and discharge.
Therefore, strong binding powers between a collector and active material and between active material particles (hereinafter briefly referred to as xe2x80x9cbinding powerxe2x80x9d when appropriate) and a good binding durability by which separation of an active material from a collector and from each other caused by volume change of active material particles at repetition of charge and discharge (hereinafter briefly referred to as xe2x80x9cbinding durabilityxe2x80x9d when appropriate) are required to minimize the capacity reduction occurring at repetition of charge-discharge cycles.
To provide a binder for electrodes having good binding durability, water-soluble polymers such as cellulosic compounds and polyethylene glycol have also been proposed (JP-A H8-273671). However, the binding power is not always sufficient and breakage of electrodes is sometimes caused.
An object of the present invention is to provide a binder for electrodes having good adhesion to an electrode collector and good retention of an active material.
Another object of the present invention is to provide a slurry for electrodes containing the above-mentioned binder for electrodes.
A still another object of the present invention is to provide an electrode for a non-aqueous electrolyte secondary battery, made by using the above-mentioned slurry for electrodes.
A further object of the present invention is to provide a non-aqueous electrolyte secondary battery, equipped with the above-mentioned electrode, exhibiting good charge-discharge cycling characteristics.
To attain the above-mentioned objects, the inventors made extensive research into a binder having good binding power and binding durability, and found that a binder comprised of a polymer having a specified proportion of vinyl alcohol recurring units can provide a non-aqueous electrolyte secondary battery exhibiting a minimized reduction of battery capacity at the repetition of charge and discharge. Based on this finding, the present invention has been completed.
In one aspect of the present invention, there is provided a binder for an electrode of a non-aqueous electrolyte secondary battery characterized by comprising a vinyl alcohol polymer having 30 to 95% by weight of recurring units expressed by the following formula (1): 
In another aspect of the present invention, there is provided a slurry for an electrode of a non-aqueous electrolyte secondary battery containing the above-mentioned binder comprising the vinyl alcohol polymer, an active material and a liquid medium.
In still another aspect of the present invention, there is provided an electrode for a non-aqueous electrolyte secondary battery, made from the above-mentioned slurry for an electrode.
In a further aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery equipped with the above-mentioned electrode made from the slurry for an electrode.